With the recent trend toward aging society, the morbidity of adult diseases is rapidly increasing. In particular, arteriosclerosis is well known as one of the primary factors causative of circulatory system diseases, such as heart diseases and cerebrovascular diseases (including brain infarction).
The accumulation of cholesterol on blood vessels plays an essential role in the growth of arteriosclerosis, and is associated with various lipoproteins in blood. Lipoproteins include cholesterol, cholesterol ester, neutral fat, phospholipid, and apoprotein as components. Of these lipoproteins, cholesterol in high density lipoprotein (HDL) is known as a negative risk factor for arteriosclerosis, whereas cholesterol in low density lipoprotein (LDL) is known as a positive risk factor for arteriosclerosis. These cholesterols are frequently observed in the field of clinical diagnosis.
In recent years, however, it has been reported that the cholesterol in a lipid metabolism-induced lipoprotein such as remnant-like lipoprotein, rather than the LDL cholesterol, is a more closely related indicator as the risk factor for arteriosclerosis. From the viewpoint of such problems, it is increasingly becoming necessary to develop a technological method for measuring cholesterols in lipoprotein in biological samples more easily and efficiently.
Among the conventionally existing methods for enzymatically measuring cholesterol in remnant-like lipoprotein, there are reports disclosing a method characterized by increasing a measurement selectivity by using a gel on which an antibody to an apoprotein as a component of lipoprotein is immobilized or by using a phospholipid-degrading enzyme, a surfactant, or the like, allowing cholesterol esterase and cholesterol oxidase or cholesterol dehydrogenase to act so that cholesterol ester is converted into cholesterol, and then measuring the produced hydrogen peroxide or reduced coenzyme (see Patent Documents 1 and 2 and Non-Patent Document 1).
Even such methods, however, have advantages and disadvantages in terms of selectivity to remnant-like lipoprotein, easiness of operation, measurement time period, and the like, and thus are not necessarily sufficient.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2001-231597.
Patent Document 2: Japanese Patent No. 2949354
Non-Patent Document 1: Domyakukoka (arteriosclerosis) 25 (9, 10), 371 (1998)